Different approaches have been proposed over the years for accomplishing 3D surface measurements. However, although many of these past approaches can be useful in some cases, none was found to be completely satisfactory for a number of reasons.
For instance, past approaches using phase-shifting techniques can offer good resolutions and yield results without requiring an extensive computation power. Phase shifting, however, generally involves moving the object surface being measured or moving a grating pattern during the data acquisition. Thus, the precision of the measurements often depends on the precision of the movements. Highly precise measurements generally entail using sophisticated moving mechanisms, thereby adding costs and complexity.
Past approaches using Moiré contouring techniques often experience limitations in the range of measurements because of the modulo 2π ambiguity. The measurement precision can decrease if the measurement range increases. Some strategies have been suggested to mitigate this phenomenon. However, these strategies are often based on the assumption that the object surface is devoid of abrupt discontinuities.
Past approaches using color coding with projections of color stripes or color dots are generally considered satisfactory but can have a limited precision since 3D measurements are often gathered from a relatively small number of color stripes or dots. Still, variations in the reflectivity of the object surface can have an impact on the measurements since reflectivity is often assumed to be uniform.
Another example of a past approach is the one using of fringe projections. These fringe projections can be generated using various devices, such as LCD (liquid crystal display), DLP (digital light processor) or other dynamic light modulation devices. These various devices dynamically produce different patterns and can be adapted to many different situations. However, their speed is limited by the response time of the dynamic light modulation device. By contrast, a projection through an optical filter is limited by the light source response time. The spatial and intensity resolutions of a dynamic light modulation device are often inferior to that of a projection through an optical filter.
Some other past approaches involve the projection of patterns having continuously varying wavelengths. This way, a direct relationship is created between the projection angle and the detected wavelengths reflected on the object surface. However, the precision of the measurements often depends on the precision of the detector sensing the reflected wavelengths. If some range of wavelengths cannot be detected, for example when using a RGB (red, green and blue) detector, the precision can decrease since some information will be lost.
U.S. Pat. No. 6,559,954, issued 6 May 2003 to Takata et al., discloses a method and a device in which multiple images containing a plurality of wavelength bands are projected, a pattern being located in each wavelength band of each image. This method and device necessitate dynamic spatial light modulators, such as found in a conventional color projector, which may not allow achieving short acquisition time, for instance of less than 1000 μs, to gather accurate 3D data from the surface of a moving object.
U.S. Pat. No. 6,937,348, issued 30 Aug. 2005 to Geng, discloses a method and a device in which a single image containing a plurality of wavelength bands is projected, the image containing a pattern located in each of the wavelength bands in order to obtain three-dimensional information. This method and device, however, may not always yield optimum results if the surface reflectivity in not at least approximately known in advance and if the surface reflectivity varies depending on the wavelength.
Accordingly, room for improvements still exists in this area.